Dehydrocyclodimerization (DHCD) is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function.
Since the product stream from the dehydrocyclodimerization process contains a mixture of compounds, it must undergo several separation steps in order to obtain usable products. An initial fractionation will separate the C.sub.6.sup.+ products from uncondensed materials which include fuel gas, hydrogen and unreacted hydrocarbons. The uncondensed material is compressed and sent to a gas recovery section where hydrogen and fuel gas are separated from the unconverted hydrocarbons which are recycled to the dehydrocyclodimerization zone.
Significant fixed and operating costs are associated with the product compressor and gas recovery equipment. The high compression ratio required to condense the materials accounts for the substantial capital and operating costs. Therefore, it would be desirable to eliminate the high costs associated with the compressor and gas recovery equipment.
Applicants have found a solution to this problem which involves dehydrogenation. The uncondensed materials from the product fractionator are flowed to a dehydrogenation zone where the unreacted hydrocarbons (C.sub.3 /C.sub.4) are converted to olefins. Since the operating pressure of the dehydrogenation zone is lower than that of the DHCD zone, there is no need for a compressor. The effluent stream from the dehydrogenation zone is separated into a hydrogen stream, a C.sub.1 /C.sub.2 fuel gas stream, a C.sub.3.sup.= /C.sub.4.sup.= olefin stream and an unreacted hydrocarbon stream (compression is required for this separation). The unreacted stream is recycled to the DHCD zone, while the olefin stream is collected and the other streams are vented.